Friction wedges, so referred to because of their general shape, but also referred to as friction castings, are used in a wedge-shaped bolster pocket ("pocket" for brevity) of a railroad car truck ("truck" hereafter) to damp oscillations of springs supporting the truck's bolster (bolster). Such friction castings are conventionally made of cast iron, cast as a unitary article, or, are made by combining a cast iron body with a wear plate or "pad" of chosen material. Use of a wear plate is taught in U.S. Pat. Nos. 3,559,589 to Williams and to 4,426,934 Geyer; use of twin pads of polymer, aptly positioned in the pocket is taught in U.S. Pat. No. 4,974,521 to Eungard, the disclosure of which is incorporated by reference thereto as if fully set forth herein; and, each of the foregoing, inter alia, illustrates the stabilizing function of a friction wedge.
A unitary friction wedge is typically cast as a single metal body, preferably of acicular cast iron or cast steel, and, as it is held in the pocket, presents a slanted planar face, slanted at an angle in the range from 50.degree. to 60.degree. to the horizontal, a vertical face (plane forming the y-axis), and a horizontal bottom face (x-axis plane). In a combination friction wedge, a pad means, namely, one or more pad members, is secured on the slanted supporting surface of a metal body support member of a wedge-shaped cast iron body, and the combination friction wedge is positioned in the pocket such that the pad's exposed surface abuts the slanted surface of the pocket; and, the vertical face of the cast iron body is biased against a guide column of the truck's side frame (hence referred to herein as a `friction casting`). In the present invention, the pad means is a synthetic resin or polymer, having specified properties which provide the friction wedge with safe, reliable and long-lived service under operating conditions.
The problem is to exert the appropriate amount of friction force in reaction to, and as a function of, the forces exerted by the truck while the car is in motion, such that the "ride" of the car is controlled within predetermined limits. This problem is satisfactorily solved with a conventional acicular unitary cast iron friction casting, except that the slanted cast iron surface of the friction casting causes an excessive amount of wear on the inclined bottom wall of the bolster which is also the inner surface of the rear wall (provided by the end of the bolster) of the pocket due to the abrasive effect of the harder (than hard steel) cast iron, on the hard steel of the bolster. However, deformation under load and thermal stability were not problems when a unitary cast iron friction casting was used. Therefore, to use a polymeric material successfully, the problem was narrowed to finding a substitute material not only with better non-abrasive properties, or, stated differently, better lubricity, but with acceptably minimal compressive deformation even at the elevated temperature conditions to which a truck is subjected during operation.
Still further, the material chosen was to have characteristics which lent it to being shaped precisely, with conventionally available techniques, economically, and which allowed the shaped material to maintain its shape during operation, over time.
Thus, the present invention specifically seeks to emulate the satisfactory performance of the unitary cast iron friction wedge against the hard steel of the bolster, by substituting a combination friction wedge in which the slanted surface is provided by a specific type of known synthetic resinous materials (polymers). Such a polymer has better lubricity (lower coefficient of sliding friction) than a material suggested for such use in the prior art, does not deform appreciably even at elevated temperature, and has great tensile strength and resistance to impact.
Half a century or more ago, friction wedges with a resilient pad member were known to be desirable, as disclosed in U.S. Pat. Nos. 2,053,990 to Goodwin; 2,333,921 to Flesch; 2,693,152 to Bachman; and, more recently in U.S. Pat. Nos. RE. 31,784; 4,295,429 and 4,915,031, to Wiebe, inter alia. Despite the deficiencies of the resilient pad members disclosed in the prior art, Wiebe in the '031 patent, requires that a pad of his friction casting be formed from an elastomer which provides a particular "stick-slip" action (see col 2, lines 18-19) distinct from the abrupt action believed to be provided by other friction castings such as cast iron unitary wedges.
In particular, Wiebe taught that the elastomeric friction elements disclosed in the '784 and '429 patents offer improved damping for all modes of relative bolster to side frame motion, and that preferably, such elastomer means be combined with a friction casting having a metal body. Further, that overstraining portions of an elastomeric element causes those portions to take a permanent set and/or lose some of their resiliency or fail structurally; and that when the resiliency of an elastomeric element loses uniformity, its ability to operate as described in the identified patents may be substantially impaired (see col 3, lines 9 et seq).
From the foregoing it is clear that the teaching of an elastomeric element requires that it not only be an elastomer, but that the elastomer have desirable resiliency such that overstraining and overheating of the elastomer will not cause its deterioration. Flesch '921 teaches that the resilient pads he uses are compressed between friction shoes and wedge members whereby the compression of the resilient pads remains substantially unchanged upon relative vertical (y-axis) movement of the bolster and side frames. While he states "any suitable resilient material" (col 3, lines 32-33) may be used, he requires that "any longitudinal movement of the bolster with respect to the side frame will be cushioned by further compression of the rubber pads." (see col 4, lines 14-17). Clearly the pads are compressible.
Though the references all refer to a resilient material having the desirable characteristics to meet the demands of an operable friction wedge, they make it abundantly clear that choosing the "right" material to meet the exigent performance specifications of a friction wedge, is a difficult task which does not lend itself to routine trial and error experimentation such as one skilled in the art would be expected to undertake. The material must not only have the friction characteristics desired, but those properties must be available over the operating range of temperature and pressure exerted by a railroad car in operation. Further, a synthetic resinous material chosen must not abrade excessively, nor cause excessive wear in the pocket, nor deteriorate over the expected service life of the railroad car, all problems which, in most materials, is exacerbated by thermal and oxidative degradation due to the material being heated under conditions normally encountered in operation of the car.
At the present time, friction wedges are commercially available in which the pad member is formed from the following materials: ultrahigh molecular weight (UHMW) polyethylene (PE) disclosed in the '521 patent; cast polyurethane having microscopic voids &gt;10 .mu.m (micrometers), usually &gt;20 .mu.m, characteristic of cast crosslinked polymers; and, cast molybdenum-filled polyurethane (UMF) having a Shore D hardness of less than 70 (&lt;70 Shore D). Each of the prior art materials suffers from unacceptable thermal degradation under load, and, a sliding coefficient of friction which is in the range above 0.2, typically about 0.3, as measured in a test in which moduli values are calculated to represent effective values for the "in situ" or installed pad configurations as developed in a test used by Engineering Systems Inc. and designed for the purpose by Robert W. Bullock. (see ESI File No. 1651 A).
It should be noted that a pad placed on the slanted face of a friction casting provides an insulator in one of the primary paths for conduction to dissipate heat generated. As a result, the temperature of the pad can remain in the range from about 93.degree.-149.degree. C. (200.degree.-300.degree. F.) for extended periods of time during operation of a car on hot Summer days.
Specifically, prior art polymer pads suffer &gt;5% compressive deformation (or, are strained more than 5%) at 177.degree. C. (350.degree. F.) under pressure of 6890 kPa (1000 psi), and &gt;1% compressive deformation under pressure of 6890 kPa (1000 psi) and a temperature of 38.8.degree. C. (100.degree. F.) and poor tensile strength. As a result, pad members made from the prior art materials are found to require premature replacement at the end of only one year, when the pad members are used in 90.7 metric ton (100-ton Avdp US) flat cars carrying heavy machinery from a manufacturing plant to a shipping site, in dedicated service. In contrast, a reaction injection molded (RIM) polymer which is essentially non-deformable as defined herein, and particularly such a RIM polymer having a minor amount by weight of a polyolefin disperse phase, provide pads which have an unexpectedly advantageous combination of lubricity and lack of compressive deformation; and, are surprisingly long-lived in 100-ton flat-cars used under substantially the same conditions as those used to test the prior art friction wedges.
It is further believed that the absence in the RIM pads, of microscopic voids 20 .mu. in nominal diameter, such as are generally present in a cast polymer matrix, unexpectedly provides the pads not only with (i) great tensile strength surprisingly greater than that of cast polymer, and (ii) essentially no compressive deformation, but also with (iii) hysterisis characteristics which approach the energy loss of acicular cast iron.
In the invention described in greater detail herebelow, it is essential that the polymer chosen for use as a pad, be non-elastomeric, essentially incompressible under normal loads, even at a temperature as high as 177.degree. C., or a railroad car will neither provide satisfactory operation, nor safe and reliable service over its expected service life. As will presently be evident, it is essential that a substantially rigid polymeric pad be used, and that the pad, as a component of a friction wedge be essentially non-deformable under the conditions of its use.
By "substantially rigid" is meant that the polymeric pad used herein, when subjected to a distortion force normally encountered within the environment of a bolster pocket at ambient temperature, and associated with the securing operation of an assembly of springs between the bolster and the side frame, is capable of resisting the distortion force applied to the pad as it is oriented in the pocket, and capable of maintaining the wedge's formational shape thereafter.
The term "elastomer" is used herein in its accepted meaning to refer to a polymeric material such as a synthetic rubber or plastic, which at room temperature can be stretched under low stress to at least twice its original length and upon immediate release of the stress returns with force to its approximate original length (McGraw Hill Dictionary of Scientific and Technical Terms, pg 648, 5th Edition, McGraw Hill Book Co.) The phrase "sufficiently crosslinked to provide a substantially rigid matrix" is used to refer to a RIM polymer which has the physical properties described below.
By "essentially non-deformable" and "essentially no compressive deformation" is meant that the material has a compressive deformation of less than 1% at 38.degree. C (100.degree. F.), and more importantly, &lt;5% at 177.degree. C. (350.degree. F) under a load which produces about 6890 kPa (1000 psi) pressure, indicating the material is essentially incompressible in the stated temperature range under the operating conditions for a truck.
Though the wedge-shaped pocket is conventionally used in railroad cars in this country, the invention herein may be adapted for use in a bolster pocket of arbitrary shape, so long as the inner surface of the rear wall of the pocket has a proclivity to wear undesirably due to the continuous vibrations to which the bolster is subjected while a car is in operation. More specifically, friction wedges of this invention are shaped as described in the aforementioned '031 or '521 patents. Such shapes include a unitary generally rectangular pad between the slanted surface on the body of a friction casting and the inner surface of the rear wall of the pocket, and, a pair of generally rectangular pads, or pads molded with an arcuate rear surface to fit on a support portion of a friction casting, are used. Twin pads may be self-adjusting during use, so they make full contact with the pocket's slanted rear wall and adjoining side walls.